This invention is directed to the method and apparatus for removing phase distortion from seismic traces or data, which method and apparatus are particularly applicable for removing phase distortion from deconvolved seismic data, and, still more particularly, from seismic data which has been deconvolved with a minimum phase inverse operator.
In seismic prospecting a seismic energy source is employed to generate a seismic signal which is transmitted into the earth. A portion of this signal is reflected from reflecting interfaces within the earth back toward the surface of the earth where it is received by detector stations positioned to receive the reflected signals. The detector stations are usually comprised of a group of geophones which generate electrical signals in response to received seismic signals. The geophones are electrically connected with seismic recording instruments for recording the electrical signals. A recording of one seismic channel is referred to as a trace or seismic trace. When the trace is recorded in analog form on a magnetic tape there is a continuous record written on the tape of an output voltage of a seismic amplifier used in conjunction with the recording of the electrical signals generated by the geophones. When processed in analog form these electrical signals are operated upon for example by filtering to present them in form for use by geophysicists. With the event of digital recording of seismic data a discontinuous record of the signal is written that measures the seismic amplifier output voltage only at discrete intervals. The digitally recorded data may be operated upon to present the data in form for use by geophysicists. Reference may be had to "A Pictorial Digital Atlas", 1966 Edition, prepared by Staff Members of United Geophysical Corporation, and presented at the 36th Annual Meeting of the SEG, Houston, Texas, November 1966, for a better understanding of digital recording and analysis and processing of seisimic signals.
There are many different types of seismic energy sources employed in seismic prospecting. In general, these energy sources may be classified in two general classes, the first being those by which an impulse signal is transmitted into the earth, for example by the use of explosives; and the second being those by which vibrational energy is transmitted by a continuous or semicontinuous process into the earth, for example by the use of hydraulic vibrators to generate a wave train of controlled frequencies.
In U.S. Pat. No. 2,688,124 to Doty et al. there is described a method of determining the travel time, between spaced first and second points, of a unique signal which is non-repetitive during a period which is at least as long as such travel time. The steps of this method are as follows:
(a) transmit such a signal from the first point, PA1 (b) provide a counterpart of the transmitted signal, PA1 (c) multiply at least a substantial portion of the total transmitted vibration energy which is received at the second point by the counterpart signal, PA1 (d) integrate for a substantial period the product of the multiplication, and alter the phase relation between the counterpart signal and the transmitted signal during successive integration periods, and PA1 (e) record the values derived from the integration whereby the out-of-time phase relation of the counterpart signal with respect to the transmitted signal at the first point, which yields the greatest value from such integration, may be used as the parameter of the travel time of the unique signal between the points.
Doty et al. teaches that the energy source or tranducer used to generate the initial unique signal which is non-repetitive for substantial periods may be of any type which can transmit a harmonic compressional wave into the earth, for instance, with a controllable frequency and substantially constant amplitude.
In U.S. Pat. No. 3,689,874 to Foster et al. there is described an invention which relates to the processing of geophysical data to compensate for the effect of distortion introduced in obtaining the data and, more particularly, relates to a type of inverse filtering wherein the geophysical data are processed to obtain the inverse filter and wherein the inverse filter is then applied to the geophysical data in a manner which broadens the frequency band of the geophysical data. Geophysical data includes distortion which often obscures the desired characteristics of the geological formations and in seismic exploration this distortion often takes the form of multiple reflections, commonly referred to as ghosts or reverberations.
Foster et al. characterized the distortion present in seismic data by an operation which separates the distortion component from the component representing the characteristics of the geological formations. The inverse of the distortion filter is then obtained and this inverse distortion filter is applied to the geophysical data to produce an output which emphasizes the true characteristics of the geological formations. This output is applied to a filter which has specified desirable properties such as a broad flat signal spectrum in the seismic band. This filter, which is referred to as the D filter, renders the final output a good estimate of the geophysical data which would have been produced in the absence of the distortion.
Foster et al. further teaches how to characterize the distortion present in a seismic trace and then, having characterized the distortion, how to generate an inverse filter which is the inverse of the distortion. Foster et al. then applies the seismic trace to the inverse filter and obtains an output that represents primarily the reflectivity of the formations from which the seismic data was obtained. The output of the seismic data is then applied to the D filter which introduces the desired shot pulse to produce a signal which approximates the signal which would have been produced by a desired shot pulse interacting with the reflectivity of the earth in the absence of any distortion.
The technique of Foster et al. is an example of a deconvolution technique wherein seismic data is deconvolved with a minimum phase inverse operator. In deconvolving geophysical data and in particular in deconvolving geophysical data with a minimum phase inverse operator there may be introduced into the data a phase distortion. The present invention provides a technique for correcting such phase distortion in geophysical data.